1. Field of the Invention
The present invention relates to methods for determining the abundance of methane in rock samples recovered from subsurface earth formations and for characterizing the subsurface formations based upon the relative abundance of methane contained in the rock samples recovered therefrom. Particularly, the present invention relates to methods for releasing and measuring the amount of methane retained in rock samples recovered from subsurface formations and characterizing the subsurface formations according to the relative abundance of methane in the rock samples recovered therefrom. Subsurface formations characterized as containing a relatively great abundance of methane by the methods of the present invention may then be considered as prospective for containing commercial deposits of natural gas.
2. Background of the Invention
Most natural gas deposits of commercial interest are comprised predominantly of methane. Such natural gas deposits also contain varying proportions of other fluids, such as carbon dioxide, hydrogen sulfide and other low molecular weight hydrocarbons. Methane is presently a major energy source in the United States of America and in other countries, and will become a more important energy source in the future. Thus, finding additional commercial deposits of methane containing natural gas is becoming increasingly important.
Over the years, a major portion of commercially producible natural gas deposits, in high permeability subsurface formations located onshore in the United States of America, have been discovered and are being produced. Exploration activity is now being directed to locating commercially producible natural gas deposits in lower permeability subsurface formations. It is expected that most future natural gas reserves found onshore in the United States of America will be found in subsurface formations having permeabilities of about 4 millidarcies, (md), or less. Formations having permeabilities as low as 0.001 md have been economically produced. With the depletion of existing natural gas reserves, new natural gas reserves in low permeability subsurface formations will have increasing economic importance.
Deposits of natural gas in low permeability subsurface formations are often huge, but are notoriously difficult to locate. Such deposits of natural gas in low permeability subsurface formations most often do not occur in anticlinal traps which can be reliably found using current exploratory techniques, such as seismology. And, such deposits must be sought using subsurface techniques such as well logging and analysis of drill cuttings and core samples. One such technique comprises analyzing rock samples recovered from subsurface formations during drilling operations for the presence of natural gas. Such rock samples commonly comprise drill cuttings and core samples. The rock samples are analyzed to determine relative abundances of natural gas therein and thereby identify subsurface formations which are prospective for containing commercial deposits of natural gas. Subsurface formations identified as containing relatively great abundance of natural gas, based upon relative abundance of natural gas in rock samples from the subsurface formations, are then subjected to additional technical and economic methodologies, such as geophysics, structural studies, stratigraphy, land acquisition and pipe line access, for selecting the subsurface formations which are prospects for containing commercial deposits of natural gas.
Natural gas is contained within pores of the rock matrix which comprise subsurface formations. The size of such pores varies considerably, from the millimeter range and larger in high permeability formations to the millimicron range or smaller in low permeability formations. The smaller the pores, the tighter natural gas is held within the rock matrix of a formation. Natural gas readily escapes from rock samples recovered from high permeability subsurface formations. However, natural gas does not readily escape from rock samples recovered from low permeability subsurface formations, and may be retained in the rock samples for extended periods of time. It is well known to analyze rock samples from low permeability subsurface formations to determine the relative abundance of hydrocarbon fluids, including natural gas, in such rock samples.
Known methods for analyzing rock samples to determine hydrocarbon fluid content include physical crushing or thermally decrepitation of rock samples under conditions of reduced pressure, or vacuum, for releasing hydrocarbon fluids as vapors. The hydrocarbon vapors are then analyzed, by means such as mass spectrometry or gas chromatography, for determining the relative abundance and composition of hydrocarbon fluids in the rock samples. The cost of equipment required for such analyses is high, as is the cost for housing and operating the equipment. Also, for low permeability rock samples, crushing the rock samples will not release substantial portions of the natural gas and other hydrocarbons from the small, (millimicron range), pores. Further, heat decrepitation of rock samples tends to produce extraneous gasses as a result of thermal degradation of inorganic minerals and thermal cracking of heavier hydrocarbons in the rock samples.
A method for rapidly and inexpensively determining the relative abundance of methane in rock samples from low permeability subsurface formations, and for the subsequent characterization of the subsurface formations based upon the relative abundance of methane in the rock samples will be particularly useful for identifying subsurface formations which are prospects for containing commercial deposits of natural gas.
Now, according to the present invention, a method is disclosed for determining the abundance of methane in rock samples recovered from subsurface formations, which method comprises:
a) selecting a rock sample recovered from a subsurface formation, for testing to determine the abundance of methane therein;
b) completely submerging the rock sample in an aqueous acid solution within a container having interior walls and bottom, for a time sufficient to dissolve at least a portion of the rock sample and releasing natural gas from the rock sample;
c) determining the amount of gas contained in bubbles retained in the container as a measure of the abundance of methane contained in the rock sample.
The method of the present invention further comprises:
d) characterizing the subsurface formation from which the rock sample was recovered based upon the relative abundance of methane in the rock sample.
The method steps a) through d) may be repeated for a plurality of rock samples from a plurality of subsurface formations and each of the subsurface formations characterized based upon the relative abundance of gas bubbles, (methane), released from the rock sample recovered from each subsurface formation.
According to the present invention, I have discovered that gas bubbles, released from a rock sample by reaction of aqueous acid solution and retained in the container, substantially comprise methane. Thus, the volume of gas in such bubbles is an indication of the abundance of methane contained in the rock sample treated according to the method of the present invention.